ASTM D7235-21a

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Designation: D7235 − 21 D7235 − 21a
Standard Guide for
Establishing a Linear Correlation Relationship Between
Analyzer and Primary Test Method Results Using Relevant
ASTM Standard Practices
This standard is issued under the fixed designation D7235; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Operation of a process stream analyzer system typically involves four sequential activities:
(1)Analyzer Calibration—When an analyzer is initially installed, or after major maintenance has been
performed, diagnostic testing will typically be performed to demonstrate that the analyzer meets
manufacturer’s specifications and historical performance standards. These diagnostic tests may require
that the analyzer be adjusted so as to provide predetermined output levels for certain reference
materials. (2)Correlation to Primary Test Method—For process stream analyzer systems where the
application objective is to provide prediction of results from a Primary Test method, once the
diagnostic testing is completed, process stream samples will typically be analyzed using both the
analyzer system and the corresponding primary test method. A mathematical function will be derived
that relates the analyzer output to the primary test method (PTM). The application of this mathematical
function to an analyzer output produces a predicted PTM result. (3)Initial Validation—Once the
relationship between the analyzer output and primary test method results has been established, an
initial validation is performed using an independent data set to demonstrate that the predicted PTM
results agree with those from the primary test method within the tolerances established from the
Correlation activities and with no statistically observable systemic bias. (4)Continual Validation—
During normal operation of the process analyzer system, quality assurance testing is conducted to
demonstrate that the agreement between analyzer and primary test method results during the Initial
Validation is maintained. This document provides guidance for item (2) above.
1. Scope*
1.1 This guide covers a general methodology to develop and assess the linear relationship between results produced by a total
analyzer system versus the results produced by the corresponding primary test method (PTM) that the analyzer system is intended
to emulate, using the principles and approaches outlined in relevant ASTM standard practices and guides.
1.2 This guide describes how the statistical methodology of Practice D6708 can be employed to assess agreement between the
PTM and analyzer results, and, if necessary, develop linear correlation to further improve the agreement over the complete
operating range of the analyzer. For instances where there is insufficient variation in property level to apply the Practice D6708
multi-level methodology, users are referred to Practice D3764 to perform a level specific bias evaluation. The correlation
relationship information obtained in the application of this guide is applicable only to the material type and property range of the
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.25 on Performance Assessment and Validation of Process Stream Analyzer Systems.
Current edition approved July 1, 2021Dec. 1, 2021. Published July 2021December 2021. Originally approved in 2005. Last previous edition approved in 20162021 as
D7235 – 16.D7235 – 21. DOI: 10.1520/D7235-21.10.1520/D7235-21A.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7235 − 21a
materials representative of those used to perform the assessment. Users are cautioned against extrapolation of the relationship
beyond the material type and property range being studied.
1.3 This guide applies if the process stream analyzer system and the primary test method are based on the same measurement
principle(s), or, if the process stream analyzer system uses a direct and well-understood measurement principle that is similar to
the measurement principle of the primary test method. If the process stream analyzer system uses a different measurement
technology from the primary test method, provided that the calibration protocol for the direct output of the analyzer does not
require use of the PTM, this practice also applies.
1.4 This guide does not apply if the process stream analyzer system utilizes an indirect or mathematically modeled measurement
principle such as chemometric or multivariate analysis techniques where results from PTM are required for the chemometric or
multivariate model development. Users should refer to Practices D8321 and D6122 for detailed correlation and model validation
procedures for these types of analyzer systems.
NOTE 1—For example, this guide would apply for the comparison of benzene measurements from a mid-infrared process analyzer system based on Test
Method D6277 to those obtained using PTM Test Method D3606, a gas chromatography based test method. For each sample, the mid-infrared spectrum
is converted into a single analyzer result using methodology (Test Method D6277) that is independent of the primary test method (Test Method D3606).
However, when the same analyzer uses a multivariate model to correlate the measured mid-infrared spectrum to Test Method D3606 reference values
using the methodology of Practice D8321, this guide does not apply. In this case, the direct output of the analyzer is the spectrum, and the conversion
of this multivariate output to an analyzer result require results from the primary test method.
1.5 This guide assumes that the analyzer sampling system is fit for use, and both analyzer and lab systems are in statistical control
during the execution of the required tasks. Procedures for testing for proper function of the analyzer sampling system are beyond
the scope of this guide. For ascertaining whether the systems are in statistical control, refer to Practice D6299 or other technical
equivalent documents.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D3606 Test Method for Determination of Benzene and Toluene in Spark Ignition Fuels by Gas Chromatography
D3764 Practice for Validation of the Performance of Process Stream Analyzer Systems
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D5191 Test Method for Vapor Pressure of Petroleum Products and Liquid Fuels (Mini Method)
D6122 Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared
Spectrophotometer, and Raman Spectrometer Based Analyzer Systems
D6277 Test Method for Determination of Benzene in Spark-Ignition Engine Fuels Using Mid Infrared Spectroscopy
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
D6624 Practice for Determining a Flow-Proportioned Average Property Value (FPAPV) for a Collected Batch of Process Stream
Material Using Stream Analyzer Data
D6708 Practice for Statistical Assessment and Improvement of Expected Agreement Between Two Test Methods that Purport
to Measure the Same Property of a Material
D7453 Practice for Sampling of Petroleum Products for Analysis by Process Stream Analyzers and for Process Stream Analyzer
System Validation
D7808 Practice for Determining the Site Precision of a Process Stream Analyzer on Process Stream Material
D8321 Practice for Development and Validation of Multivariate Analyses for Use in Predicting Properties of Petroleum
Products, Liquid Fuels, and Lubricants based on Spectroscopic Measurements
E1655 Practices for Infrared Multivariate Quantitative Analysis
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D7235 − 21a
2.2 American Petroleum Institute Document:
API RP-555 Process Analyzers
3. Terminology
3.1 All of the terminology as defined in Practices D3764, D6122, and D6708 are adopted for this guide.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 primary test method (PTM), n—test method designated by the user of this guide such that the process analyzer system results
are estimates or prediction of PTM results that would have been obtained if applied to the same material.the analytical procedure
used to generate the reference values against which the analyzer is both calibrated and validated. D3764
3.2.1.1 Discussion—
It is not the intent of this guide to define a test method. Within the context of the intended application of this practice, a PTM can
be any user-designated test method that the process analyzer system test results are intended to estimate or predict.
4. Significance and Use
4.1 This guide is intended to be used in conjunction with Practice D3764 (Case 1) and Practice D6122 (Case 2). Methodology
in this guide can be used to determine if a linear correlation can improve the performance of the total analyzer system in terms
of its ability to predict the results that the PTM would have been if applied to the same material. This methodology, which is based
on the same statistical data treatment as Practice D6708, is used to derive the parameters of the linear relationship and to assess
the degree of improvement.
4.2 This guide provides developers or manufacturers of process stream analyzer systems with useful procedures for developing
the capability of newly designed systems for industrial applications that require reliable prediction of measurements of a specific
property by a primary test method of a flowing component or product.
4.3 This guide provides purchasers of process stream analyzer systems with some reliable options for specifying performance
requirements for process stream analyzer systems that are used in applications requiring reliable prediction of measurements of a
specific property by a primary test method of a flowing component or product.
4.4 This guide provides the user of a process stream analyzer system with useful information on the work process for establishing
the PTM prediction relationship and prediction performance.
4.5 Prediction (correlation) relationship obtained in the application of this guide is applicable only to the material type and
property range of the materials used to perform the study. Selection of the property levels and the compositional characteristics
of the samples must be suitable for the application of the analyzer system. Users are cautioned against extrapolation of the
prediction relationship beyond the material type and property range used to obtain the relationship.
4.6 The degree-of-agreement assessment promoted in this guide is based on the statistical principles articulated in Practice D6708,
which is purely statistical in nature. No attempt is made in assessing the degree of similarity in the analytical technique between
the process analyzer and the PTM; hence, results between the PTM and analyzer unit can be highly correlated, but their
measurement principles may be completely different, and may not be the principal cause for correlation. Users are therefore
cautioned that a high degree of correlation between results does not necessarily imply a high degree of similarity in the
measurement principles; nor does it imply a similar degree of agreement can be expected in future measurements. In general, if
sample-specific biases are detected, it suggests that the measurement principles may be different, and may affect the
degree-of-agreement in future use of the scaling/bias-correction equation. Presence or absence of sample-specific effect can be used
as a measure of the robustness of the correlation equation to sample composition or matrix differences.
4.7 Implementation of this guide requires that the process stream analyzer system complies with the following conditions:
4.7.1 Meets the principles set forth in Process Analyzers of API RP-555,
4.7.2 Meets the supplier’s recommendation,
Available from American Petroleum Institute (API), 1220 L. St., NW, Washington, DC 20005-4070, http://www.api.org.
D7235 − 21a
4.7.3 Complies with operating conditions specified by the manufacturer,
4.7.4 A predicted PTM algorithm has already been established if necessary, and
4.7.5 Meets applicable quality assurance, data collection and data telemetry protocols.
4.8 After installation or major maintenance, conduct such diagnostic tests as recommended by the manufacturer to demonstrate
that the analyzer meets manufacturer’s specifications, historical performance levels or both. If necessary, adjust the analyzer system
components so as to obtain recommended analyzer output levels for specified reference materials.
4.9 Inspect the entire analyzer system to ensure it is installed properly, is in operating condition, and is properly adjusted after
completion of the initial commissioning procedures.
5. Analyzer Calibration Adjustments and Diagnostics Tests
5.1 When an analyzer is initially installed or after major maintenance has been performed, diagnostic tests should be conducted
to demonstrate that the analyzer meets manufacturer’s specifications and historical performance standards. These diagnostic tests
may require that the analyzer be adjusted so as to provide predetermined output levels for certain reference materials. Such
adjustment may be done in hardware, software or both. This should not be confused with the development of correlation to a PTM,
which, is described below.
6. Correlation to Primary Test Method
6.1 General Approach:
6.1.1 Define the sample set to be used for assessment.
6.1.1.1 The material type and property range for which the analyzer system results are to be assessed versus the primary test
method is
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